Many types of fiber-based communication systems rely on the use of polarization-sensitive components to perform a variety of functions. These components include, but are not limited to, polarization-diversity optical transmitters and receivers, polarization-sensitive optical modulators, amplifiers, lengths of polarization-maintaining (PM) fiber and the like. In order to maintain the polarization integrity of the systems, these components are often interconnected using PM fiber. In particular, PM fiber is optical fiber in which the polarization of linearly polarized light waves launched into the fiber is preserved during propagation, with little or no cross-coupling of optical power between the polarization modes. FIG. 1 is an isometric view of an exemplary section of PM fiber 1, which in this case utilizes stress rods 2 surrounding a core region 3 to create “stress-induced birefringence” and maintain the propagation of a linearly polarized signal along the core. As shown, the x-axis of fiber 1 is denoted as the “fast axis” and the orthogonal y-axis is denoted as the “slow axis”, with the optical signal propagating along the longitudinal z-axis. If the polarization of an incoming optical signal is not aligned with the fast (or slow) axis, the output will vary between linear and circular polarization (and generally will be elliptically polarized). The exact polarization will then be sensitive to variations in temperature and stress (including bend-induced stress) in the fiber. Additionally, the polarization maintaining properties of the optical fiber may degrade over time due to environmental effects (e.g., heat, stress, tight bends, etc.).
A good measure of the resulting polarization integrity of an optical signal propagating in such an environment is the “polarization extinction ratio” (or PER), which is a ratio of the power of the optical signal propagating along the “fast axis” with respect to the power of the optical signal propagating along the “slow axis” of the same fiber. Indeed, it is useful to measure the PER in order to correctly launch an incoming (polarized) optical signal into polarization-maintaining components, as well as monitor the ability of the fiber to preserve polarization. In this case, “correctly launch” refers to providing the appropriate alignment of the polarization axes of the signal to the axes of the system. In one form, the PER can be expressed as follows:PER=10*log10(Pfast axis/Pslow axis),  (1)where, in this particular form, the relationship is measured in decibels (dB). The PER may also be referred to, at times, as the “power splitting ratio”, since it is the ratio of power between the fast and slow axes of the optical power in the propagating signal.
In situations where the optical signal is propagating within a non-polarization maintaining optical fiber or waveguide, the PER can be expressed in a similar form and calculated by measuring the power of the optical signal along orthogonal axes.
There are prior art techniques for measuring PER of an optical system, but these techniques require that the system's optical fiber be coupled into a stand-alone device that performs the measurement. This is not always convenient, particularly after a system has been deployed in the field and it is desired to obtain an updated measurement of the signal's PER. Some of these prior art techniques require that a section of optical fiber be stretched or heated; again, this is not always possible. Moreover, these techniques require that a set of measurements be made over time and the PER be calculated from multiple measurements (e.g., as the fiber continues to be heated, measurements are performed at each temperature). Additionally, since a number of separate measurements must be performed over time to calculate the PER, it is not easy to obtain updated measurements—a significant limitation of the prior art.
Thus, a need remains in the art for a means of measuring the PER of an optical signal which does not require termination of the system PM fiber in stand-alone measurement equipment (i.e., takes the form of an “in-line” arrangement), while also able to derive the PER from a single calculation in real time (i.e., an “instantaneous” measure of the polarization extinction ratio).